The invention relates to the manufacturing of boat hulls, and more particularly to the manufacturing of fiber-reinforced polymer or fiber-reinforced plastic boat hulls.
The spray-up technique, lay-up technique, or a combination of the two is the most common method of manufacturing fiber-reinforced polymer (FRP) boat hulls. In both techniques, a mold release agent (wax) is applied to an open, single-sided, female mold, and then a layer of gelcoat, which becomes the outer surface of the hull, is sprayed onto the mold. Typical gelcoat thicknesses are from 0.4 mm to 0.8 mm. After the gelcoat has hardened, a layer ("skincoat") of chopped glass fiber and polyester resin with catalyst is sprayed onto the gelcoat with a thickness of approximately 2 mm. The skincoat is allowed to cure before the remaining layers (the bulk laminate) of the hull are applied. The purpose of the skincoat is to protect the gelcoat from the high temperatures which may result from the exothermic reaction of the resin in the bulk layer as the resin cures. The lay-up technique involves placing glass mat or other reinforcing materials in the mold and saturating the reinforcement material with a mixture of liquid polymer resin and catalyst. In the spray-up technique, chopped glass fibers, liquid resin and the catalyst are sprayed into the mold. In both methods, air and excess resin are manually forced out of the reinforcing material using hand-held rollers. Layers are added and allowed to cure as needed to build the hull to the desired thickness. Finally, wooden stringers are placed in the hull in the mold and additional layers are added over the stringers to secure the stringers to the FRP shell.
The resin most commonly used is polyester. Polyester resins used in boat building may contain styrene monomer in varying amounts. The styrene monomer reduces the viscosity of the liquid polyester resin, improving its flowability, and the styrene also cross-links with the polyester as the resin cures, resulting in a strong final product. Disadvantageously, styrene monomer is quite volatile and is an air pollutant. The spray-up and lay-up techniques result in significant styrene emissions.
It is also known to manufacture a boat hull without stringers by resin transfer molding. See, for example, U.S. Pat. No. 4,902,215. In resin transfer molding (RTM), dry reinforcement material is placed in a molding cavity defined by one or more mold surfaces, and liquid resin is then injected into the cavity to form the FRP product. See U.S. Pat. No. 4,762,740, which is incorporated herein by reference, for further explanation of conventional resin transfer molding. In one type of resin transfer molding, one of the mold halves is a flexible bag or sheet known as a vacuum bag.